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Sat, 11 Aug 2007:

Yak Shaving: So you start out with that simple problem. But half-way through fixing it, it explodes into this whole exercise in pointless dependencies. It is a rather recent wordification (never heard of that word ? it's a perfectly cromulent word). But considering the fate of the "pre-shaved yaks" guy, who ended up saying "It's a band.", I'd say it is not quite popular enough ... yet.

Now. before I start onto the real topic - let me first say that the next release of APC will be the last release compatible with PHP 4.x. Now, what is wrong with just letting the #ifdefs stay ? That's where this snippet of code comes into play.

  apc_store("a", array(new stdclass()));

It doesn't work. Now, the problem is very simple - the original patch by Marcus only checks for objects in a very shallow way. It will detect & serialize objects which are passed to apc_store - but the check does not extend deeper into the recursive copy functions.

Symmetry: But the zval* copy functions were written to be beautifully symmetric. A copy into cache is nearly the same as copying out of it. And when I say "nearly", I actually mean that until the *_copy_for_execution() optimisations were thrown in, they were actually symmetric - in & out. But objects don't play nicely with that - because they are much more than just data.

In & Out: Objects require assymmetric caching. Storing into cache is a serialize operation, while retrieving from storage is a deserialize. This ensures that they end up with the right kind of pointers, class object initialization and that the resources they hold in their opaque boxes are properly handled. The objects have to implement their appropriate magic persistance methods.

And thus begins the Yak Shaving. I need to rewrite most of the cache copy-in and copy-out functions to handle the basic assymetry. But consider this, most of the code in there has been limited for months because of the fact that I cannot optimize on PHP data structures without breaking the symmetry.

A couple of years ago, I sat through a full hour talk by Rusty Russell about talloc(). Built on top of the trusty old malloc() calls, it simplifies memory management a lot for Samba4. So bear with me as I take a brain dump of my idea - for my very intelligent reader to poke holes in (gopalv shift+2 php net).

APC's allocation strategy is a little brain dead. To allocate 4 bytes of data, it actually requires 24 bytes of space. But much more than the space wastage, I'm more concerned about the number of lock() calls required to cache a single php file - a hello world program takes about 22 lock operations (11 locks, 11 unlocks). Yes, that's actually 22 syscalls just to cache echo "hello world";.

I've previously tried to fix it with partitioned locks. The problem with that was actually cleaning up the locks, because the extension code would have to have special cases for every SAPI - because of some bugs in PHP 5.x. So, the "if you don't succeed, destroy all evidence" principle made me throw out that idea. But the cache-copy, zend-copy separation should help me revive another approach to this.

Pools: So, now that I'm officially b0rking up APC, I could as well slap on a new pool allocator, right on top of sma_allocate - ala, talloc(). The allocation speed would skyrocket, because the in-pool allocs are sequential and do not have any fragmentation issues due to blocks in the middle being free'd. As much as allocates are important, the real advantage of this would be that I could basically speed up cache expunges by a magnitude or more. The 22 syscall cache expunge for hello world would be reduced to a potential pair of syscalls - because it would be a single free of the entire pool space.

Right now the pool is actually built up to be of the following structure.

struct apc_pool_t {
	int capacity;
	int avail;
	void *head;
	apc_pool_t *overflow;
	unsigned char data[0];

I've yet to run this through an x86_64 build, but an even multiple of int/void* should align data area right into a wordsize. And I think nearly every pool should be around 4k (i.e 4096 - sizeof(apc_pool_t)) for opcode cache and 1k for data cache. I might make the latter a runtime tuneable, just to pad the APC manual up into an entire book (just in case someone asks me to write one .. *heh*).

None of this is included in APC 3.0.15, which will exit out of the gates as soon I'm sure I'm happy with its stability. The new code will probably be an APC 3.1 release, marking the end of php4 compat & opening up the door for php6 compat.

A two line bug report which exploded into nearly two thousand lines of C code - that's just classic yak shaving.

10 If it ain't broke, break it;
20 Fix it.
30 Goto 10

posted at: 09:27 | path: /php | permalink | Tags: , ,